New antidotes to shield military and civilians from chemical agents
Research from neuroscientists at Texas A&M University may have key consequences for soldiers and victims of nerve agent attacks. D. Samba Reddy, PhD, RPh, a professor of Neuroscience and Experimental Therapeutics at Texas A&M College of Medicine, recently published two papers showing why current therapies are not able to break the chemically induced neuronal circuit that causes seizures and brain cell death—and what type of drug might be better.
Organophosphate (OP) nerve agents, used as chemical warfare weapons in combat or as bioterror agents against civilians, have severe, fast-acting effects on the body. These compounds interfere with brain chemicals that turn neurons and muscles “on” and “off.”
How it works
In a normal, healthy person, a chemical called acetylcholine acts as the “on” switch. Acetylcholine is released at the junction between neurons and muscles, and this process allows the brain to tell muscles to contract. So, every time someone wants to walk—or breathe—acetylcholine is released, which acts as the “on” switch, and certain muscles are able to contract and facilitate movement. When the body needs to stop contracting its muscles, another compound, called acetylcholinesterase, acts as the “off” switch. Acetylcholinesterase essentially cuts up the acetylcholine into small pieces so that the muscles stop contracting.
When an individual is exposed to OPs, it blocks acetylcholinesterase, the “off” switch. As a result, acetylcholine, the “on” switch, builds up in massive quantities in the brain and causes widespread nerve excitation & muscle contraction. Without an “off” switch, the brain is massively excited and muscles in the body continuously contract and cannot relax. This results in muscle spasms, convulsions, continuous seizures, respiratory arrest and eventually death. If a person is able to survive a nerve agent attack, they will likely have serious brain damage.
The current situation
Currently, there are two major therapeutic drugs that are used to reverse the effects of OP nerve agents. Diazepam and midazolam are benzodiazepine anticonvulsants that can work to prevent OP-induced brain damage and seizures, if given very soon after exposure. Both drugs are very effective antidotes when given within 30 minutes of OP exposure, but do not have much effect after an hour or two after exposure. In the context of chemical warfare and unexpected civilian bioterrorism, this is not a realistic timeline.
“Although soldiers often carry auto-injectors of a drug to use on themselves in case of a nerve agent attack, in civilian populations, the quickest midazolam could be administered after calling 911 and getting to the hospital would likely be at least 40 minutes,” Reddy said. “That is the critical time period, so any anticonvulsant antidote for these OP chemical seizures has to work even after 40 minutes. That is the goal.”
Recently, Reddy has published two papers in the journals Biochim Biophys Acta and Journal of Pharmacology and Experimental Therapeutics, co-authored by his research staff Ramkumar Kuruba and Xin Wu.
In the research reported in these articles, Reddy’s team examined two major points for diazepam and midazolam. First, they examined how efficiently each medication suppressed seizures. Second, they looked at how efficiently each drug protected against brain damage. The results of both studies showed that diazepam and midazolam were very effective when given 10 minutes after exposure. However, both medications were completely ineffective when administered at 60 minutes or 120 minutes after exposure.
“The benzodiazepines don’t control seizures at later time points, but it’s not because they aren’t reaching enough quantities in the brain,” Reddy said. “The three main reasons are the loss of target receptors, the loss of neurons and damage-induced inflammation.”
Imagine the brain is covered in billions of small red targets that serve as receptors for diazepam and midazolam. If benzodiazepines are little red molecules that only recognize the red targets, then they must find their targets to be able to have an effect on the brain. However, red OP nerve agents somehow destroy these red targets, meaning that the diazepam and midazolam cannot find their receptors.
“Benzodiazepines bind to certain receptors, and these receptors disappear in more than 50 percent of neurons within 10 to 20 minutes of seizure onset,” Reddy said. “When we administered benzodiazepines at 40 minutes, it meant that 50 percent of the benzodiazepine receptors had already vanished. The administered benzodiazepines bound to the remaining 50 percent of receptors, but the maximum effect they could produce depended on the number of receptors available, no matter how large of a dose was given.”
OP poisoning will also kill neurons, which worsens the problem of too few benzodiazepine receptors.
“Massive brain cell death will further exacerbate the problem of a lack of receptors,” Reddy said. “The cell must be alive for the drug to bind to receptors. The benzodiazepine receptors are on the main neurons. However, so many of these neurons are dead that it further reduces the number of available receptors. The loss of inhibitory interneurons, which apply strong breaks on excessive brain excitation, creates a self-sustaining seizure circuit.”
Finally, nerve agents will cause severe inflammation in the brain, which will cause more cell death and a loss of more receptors.
“When we asked ourselves ‘Why are these medications failing when they are administered later?’ it gave us the idea for producing next-generation anticonvulsants that are better than benzodiazepines,” Reddy said.
Benzodiazepine receptors, the red targets, are exclusively present in post-synaptic junctions. However, a new type of what are called GABA-A receptors, which can be imagined as green targets, are present in extrasynaptic sites. While red OP molecules can destroy the “red” benzodiazepine receptors, they will have no effect on “green” GABA-A receptors.
“These GABA-A receptors should be targets for new drugs because they will not disappear in 10 to 20 minutes after nerve agent exposure,” Reddy said. “However, it is still a reverberating circuit, because the neuronal loss leads to inflammation and loss of receptors. By targeting extrasynaptic receptors, if you control the seizures, you will stop neuronal loss. Essentially, you are breaking the circuit.”
Neurosteroids that activate extrasynaptic and synaptic GABA-A receptors have the potential to stop seizures more effectively and safely than benzodiazepines. In addition, neurosteroids may confer neuroprotection by shunting the excessive excitability and its exacerbating impact on neuronal injury and neuroinflammation, which are typically associated with nerve agent poisoning. Reddy and his team are developing innovative injectable neurosteroid products for approval by the Food and Drug Administration, which could be revolutionary for both military members and civilian victims of nerve agent attacks.
“There is an urgent need to develop next-generation anticonvulsants antidotes superior to midazolam for better treatment of OP and nerve agent poisoning,” Reddy said.
This research has been supported by the U.S. CounterACT Program, Office of the Director, National Institutes of Health and the National Institute of Neurologic Disorders and Stroke.
Sarah Elmer contributed to the writing of this article.